Head-up displays in automobiles allow a driver to view information off the windshield in the form of a virtual image situated in space above the hood of the car. A projection display situated behind the steering wheel creates the image that projects light onto the windshield, which then reflects the light towards the driver. To create a single image with no ghosts, the windshield should reflect at a single surface with minimal reflection at all other surfaces. With conventional windshields, there is a problem in that they reflect off both front and back surfaces with equal efficiency. One method of avoiding multiple images involves creating a wedge in the windshield thereby superimposing two images formed by two reflections as disclosed in U.S. Pat. No. 6,636,370. While this may be effective with low-resolution images, the disclosed approach suffers in maintaining sufficient image registration over the entire viewing pupil for higher resolution images.
Another issue concerning windshield reflection relates to polarization sensitivity. Typical windshields are angled in such a way as to deflect the projected light at an incidence angle close to Brewster's angle, whereby the reflection of p-polarized light becomes effectively zero. For this reason, s-polarized light has been favored. However, this approach renders the HUD system ineffective for use by a driver wearing anti-glare, p-polarized sunglasses. U.S. Pat. No. 6,744,478 and related patents by the same inventors consider the problem of multiple reflections and disclose methods of creating a single reflecting surface by altering the polarization state of the projected light between the first and last surfaces of the windshield. However, these disclosures do not show, teach or suggest the advantageous embodiments of the present application.
The described embodiments of this application offer low-cost, windshield designs for HUD systems that reflect projected light off a substantially single interface with p-polarized light being transmitted and reflected with minimal change in its polarization state. Creating a windshield that reflects projected light from a single surface whose polarization is eventually p-polarized is an attractive solution because of some of the issues described above.